As early as Sputnik, satellite orbits were determined by measuring the location and speed of the satellite relative to an observer on earth. For instance, by knowing the frequency of transmission of a signal emanating from a satellite, the Doppler shift in a received version of the signal can be measured. This Doppler shift information can then be used to determine the orbital trajectory of a satellite about the earth. Besides Doppler shift measurements there are many other orbit measurement data types. Another orbit measurement example is range data from one or more earth fixed stations to the satellite obtained by bouncing a signal off the satellite and measuring the two-way travel time. Yet another example of an orbit determination observation is the position and/or velocity of the satellite at an instant. All of these orbit determination examples, as well as others, will be referenced as range data.
Today, orbit information is broadcast from Global Navigation Satellites. For example, the Department of Defense broadcasts the orbits of the NAVSTAR Global Positioning System (GPS) satellites in two forms. A first form of broadcast is low precision orbit information that is broadcast as part of an Almanac of general information regarding the orbits of all GPS satellites. The almanac is broadcast from every satellite and contains low precision information regarding satellite locations and orbits that are valid for about 30 days into the future. A second form of broadcast is a high precision “broadcast orbit” which is transmitted as an individual orbit for a particular satellite. Each GPS satellite transmits a broadcast orbit, which contains refined data about its own orbit. It takes about 30 seconds to receive the refined orbit from a particular satellite. A particular distinct broadcast orbit is repeatedly transmitted over a span of two hours by a GPS satellite, and is valid for a period of approximately four hours from its initial transmission. After two hours of repeated transmission a new distinct broadcast orbit is transmitted. This process repeats every two hours.
In today's world, users would like to turn a GNSS receiver on and have it fully ready to go in a several seconds or less, rather than the 30 seconds to several minutes that it can often take to perform an “ordinary acquisition” after turning on a GPS receiver. One technique to improve acquisition speed is to utilize almanac data to fairly quickly locate and acquire the satellites. However, as almanac data does not provide precise orbital data regarding the satellites acquired, a receiver has to download the broadcast orbit from an acquired satellite in order to “learn” more precisely where the satellite is located and thus make more precise positional measurements. Such an acquisition method may be fine for a user who is, for instance, navigating an automobile. This is because the reduced accuracy (e.g. positional accuracy within several meters) that is quickly achieved using the almanac information may be adequate for a task such as navigating a car while the receiver is still trying to acquire the broadcast orbits. However this reduced level of accuracy is not suited for other tasks such as engineering work. For example positional accuracy of a couple meters would be inadequate for surveying work which requires accuracy a few centimeters or less.
A second technique for improving the speed of acquisition of satellites also reduces the amount of time required to provide highly accurate positional information to several second (e.g., 3-15 seconds). This second technique involves downloading precise orbital data regarding GNSS satellites from the Internet or some other source. The precise orbital data is used to determine the satellites in view and the frequencies on which to acquire the signal(s) broadcast from these satellites. This speeds acquisition by enabling a receiver to search for signals only from satellites that are in view. This also speeds acquisition by eliminating the requirement for a receiver to search across the large range of frequencies that the signal(s) may be received upon depending upon how Doppler shift impacts reception. Finally, this allows precise positional determinations to be made without waiting to receive the broadcast orbits from each individual navigation satellite being tracked by the GNSS receiver.
One issue with this second technique for improving the speed of acquisition is that it can be costly either to acquire the precise orbit data (for example if it must be purchased) or to support a data connection to acquire the precise orbit data. Another issue with this second technique for fast acquisition is that there are some instances where it is not possible to connect with a source for such data, either because a data connection is non-existent at a particular location or because a data connection or the data source is malfunctioning.